This setting is a bit of an obsession with Red Bull Air Race teams, because it adjusts the proportion of fuel in the fuel-to-air mixture introduced to the engine during the combustion process, and getting it right enables optimum fuel economy and engine performance. Air Racing pilots do this manually, with a red lever in the cockpit that has a range between two main settings: fully “rich” or fully “lean.” The challenge is to find the happy medium somewhere in between.
It is far from easy. The high-powered Lycoming race engines used in the Red Bull Air Race are unique to the sport, operating at full power and subject to high G for a considerable amount of time. This elevates finding the “ideal” mixture to an art.
Mastering that art is even more vital because engines in the World Championship are standardised (and in the feeder Challenger Class, the entire raceplane is standardised). Optimising fuel mixture is one of the ways a pilot can rise above that level playing field.
"If you want to win, it’s very important to get the correct mixture setting to make the engine deliver maximum power,” says Challenger Class pilot Daniel Ryfa of Sweden. “In the training before every race, I always work on that to find the right setting.”
A typical flight profile of any aircraft involves multiple stages – taxi, takeoff, climb, descent, landing... Mixture setting has an important function in all of them, and pilots will make adjustments throughout the flight to find the “ideal” setting for each stage. When it comes to the World Championship, pilots work with their teams to plan the best settings every step of the way.
When a piston-powered aircraft takes off, the mixture setting is set fully “rich,” with the highest ratio of fuel to air, because for airfields at sea level this provides optimal takeoff power. However, air density decreases with increased altitude (the air gets thinner as the aircraft climbs). As a result, there is less air to mix with the same quantity of fuel, and that leads to unburnt fuel. Running “rich” like this can cause carbon deposits to form on the spark plugs, which in turn may cause a reduction in power.
On the other hand, running rich enables the fuel to cool the engine. So upon reaching cruise altitude at the top of a climb, pilots lean the mixture, reducing the proportion of fuel until the engine starts to run rough, before gradually increasing the richness until the engine performs as expected. The resulting fuel economy is much more efficient.
“On Race Day I always start flying fully rich to keep the engine cool, and just before I’m cleared into the track I set the fuel flow that corresponds to the optimum mixture setting,” Ryfa describes.
Taking the fuel mixture to the lean side of ideal gives better fuel economy with only a small decrease in power; however, the reduced fuel flow can lead to overheating of the cylinders possibly also causing detonation. (That is abnormal explosive ignition of the fuel-to-air mixture within an internal combustion engine, which in a car produces that dreaded “knocking” sound.) In other words, the ignition is out of sync, which can affect power output, with the detonation explosions actually stripping the combustion chamber walls of a protective thermal boundary layer. If left uncorrected, this risks engine overheating and damage.
“The worst thing you can do is to get the mixture too lean,” confides Ryfa. “You lose a lot of power and the engine gets hot fast, losing even more power. This is something no race pilot wants to do!”
The flight profile of raceplanes in the Red Bull Air Race is like no other. Pilots take off, warm up, climb and, yes, even cruise – right to the hold box where they await permission to start. However, the majority of the flight is performed at low level and maximum rpm. Once cleared into the racetrack, pilots adjust the power to control the track entry speed (most often 200 knots / 370 kmh / 230 mph). They then adjust the mixture setting for the flight through and around the racetrack’s Air Gates, just 10 metres above the ground.
“They’ve got one shot to get the mixture right before entry, because they can’t readjust under the rigour of the manoeuvres they’re flying through the track and under high G,” notes Red Bull Air Race Technical Director Jim “Jimbo” Reed. “The aim is to gain maximum engine performance while at full power, which for the pilots in the World Championship is 2,950 rpm.”
Reed goes on, “However, max performance found with an ideal mixture setting is also a trade-off, as the ideal mixture ratio causes the cylinders to heat up very quickly, typically allowing a race pilot to fly only one full run in the track before having to take measures to cool the engine prior to any further runs in the track. Therefore, setting the correct mixture immediately prior to entering the track is vital for optimum performance, the fastest possible time and to enable the pilot to finish a complete run without causing the engine to heat up to the point where its power is diminishing.”
One thing that can make it a little easier for race teams to find the ideal mixture is using optimal fuel.
Dr. Alisdair Clark, Aviation Fuels Research and Development Manager at Air BP, says, “Aviation fuels were developed specifically to help pilots obtain the very best performance from aircraft and engines. Our products Avgas, which is used in the Red Bull Air Race, and Jet offer attributes to achieve this goal.”
Clark continues, “Air BP are fully engaged with the ongoing development of both Avgas and Jet on an international scale, working with industry members and looking to the future - to allow customers to fly in an environmentally conscious world and to give Air Racing pilots the best tool possible to go for the win.”